Tokkai 2000-73744 published by the Japanese Patent Office in 2000 discloses an exhaust purification device provided with a trapping catalyst which prevents nitrogen oxides (NOx) contained in the exhaust gas from being released into the atmosphere, when running the internal combustion engine at a lean air-fuel ratio.
The NOx trapping catalyst traps NOx contained in the exhaust gas of the engine when it is run at a lean air-fuel ratio, and releases the trapped NOx when the engine is run at a rich air-fuel ratio. The NOx released by the catalyst is reduced by reducing agents in the exhaust gas such as hydrocarbons (HC) and carbon monoxide (CO) that are generated due to combustion of a fuel mixture of a rich air-fuel ratio, and is discharged into the atmosphere as nitrogen (N2).
When the NOx trapping catalyst traps the NOx in the exhaust gas at a lean air-fuel ratio, it also traps sulfur oxides (SOx) in the exhaust gas. The sulfur oxides poisons the catalyst, and the NOx trapping performance declines the more the poisoning by SOx increases.
In the prior art device, the trapped amount of SOx by the catalyst is computed, and when the computed amount reaches an upper limit, the air-fuel ratio of the air-fuel mixture supplied to engine is temporarily enriched and the exhaust gas temperature is increased. Due to the increase of temperature of the exhaust gas, the SOx trapped by the NOx trapping catalyst is released, and the NOx trapping performance of the NOx trapping catalyst is recovered. This operation is termed desulphating of the NOx trapping catalyst.